1. Field of the Invention
The present invention relates to a printed circuit card with built-in heat exchanger hereinafter called a printed card, and a method for making such a card.
2. Description of the Prior Art
Problems of heat dissipation constantly arise in particular in apparatus or equipment comprising a large number of printed cards provided with electronic components. Among these components, some of them dissipate a substantial amount of heat which should be evacuated under the best possible conditions in order to maintain the said components under acceptable operating conditions.
To remedy this drawback there was proposed, for example, in French Patent Application No. 82 05253, an electric and thermal printed card comprising a heat-conducting drain. This card ensures the transfer of heat by conduction through the drain which is generally made of metal, as far as the lateral edges of the card which are connected thermally to the cold walls of the equipment by means of thermal card-guide connectors, for example. This process, although more effective than the systems of cooling by forced convection used in the past, has a certain number of drawbacks. As a matter of fact, it involves a heat conduction chain comprising a large number of links constituted by:
the coupling resistance Rc of the housing on the printed card,
the transverse conduction resistance Rt of the printed card involving the thickness of the drain, and in the case of flat-brazed components, that of the printed circuit,
the longitudinal conduction resistance R1 of the drain of the card, a resistance which is directly proportional to the dimensions of the card,
the coupling resistance Rf of the lateral edges of the drain on the cold walls of the equipment, and
the conduction resistance Rp of the frame of the equipment between the cold walls and the hot well.
Evidently, the sum of the line losses caused by each of the above links encumbers the heat balance of the whole. Furthermore, the conduction resistances R1, Rf and Rp constitute a limiting constraint for the maximum power available on the card. Likewise, the mutual influence of the heating up of the components of a given card by the bias of the heat-conducting drain produces a temperature gradient along the latter which is unfavorable to the components furthest from the lateral edges of the card.
It would be desirable to remedy this drawback by providing a printed card with built-in heat exchanger having an excellent coefficient of heat exchange between the electronic components of the card and the hot well of the system.